The performance of fuel cells using a polymer electrolyte membrane (PEM) or other comparable means of controlling the flow of ions in an electrolytic reaction can be quite sensitive to the moisture level of the membrane. Proper hydration of the membrane can be necessary for the membrane to conduct protons and thereby create the movement of charged particles that creates an electric current. Moisture can be provided by water vapor in the incoming air or fuel stream (for example hydrogen) to maintain the membrane at an appropriate and desirable level of hydration. Accurate, reliable, robust, real time measurement of water vapor concentrations is necessary to assure reliable and efficient operation of the fuel cell.